Process for producing layers and layer systems, and coated substrate

ABSTRACT

A method of producing substrates with functional layers which have high optical properties and/or a high surface smoothness, in particular a low turbidity and significantly lower roughness, is provided. The method includes a sputtering process for coating a substrate with at least one functional layer, the sputtering process being interrupted at least once by the application of an intermediate layer with a thickness of less than 20 nm.

The invention relates to a process for producing a layer system withhigh-quality optical properties and/or a high surface smoothness, and tocoated substrates with high-quality optical properties and/or a highsurface smoothness.

Processes for coating substrates with in particular optical layers toproduce optical components, such as for example mirrors or reflectors,have long been known. The optical layers have a very wide range offunctions in terms of their effect on the radiation within a definedregion of the electromagnetic spectrum.

Processes for coating substrates with in particular optical layersystems which are composed of a plurality of individual functionallayers, in particular of alternately arranged layers with high and lowrefractive indices, have likewise been known for years for a wide rangeof applications. In this context, the layer systems often act as a lightinterference film, the optical properties of which are determined by thechoice of material for the layers with high and low refractive indicesand therefore of the corresponding refractive indices, by thearrangement of the individual layers and by the choice of the individuallayer thicknesses. The choice is made substantially on the basis ofknown optical design rules and design tools according to the desiredoptical properties and also the processing properties.

In recent years, PVD (physical vapor deposition) processes and CVD(chemical vapor deposition) processes have become the main processesused to produce layers and layer systems, in particular for opticallayers and layer systems.

CVD processes are used to produce layers of refractory and other metals,carbides, nitrides and oxides. The advantage that it is possible toapply a large number of materials of virtually any theoretical densityand a good bonding strength uniformly and with a high level of purity isoffset by the disadvantage that suitable reactions do not exist forevery desired layer material, the substrate has to be able to withstandthe generally high reaction temperature and also has to be chemicallystable with respect to the reactant. In general, the pressures which arerequired during the reaction are from 10 to 100 Pa, and consequently thefree path length of the particles is relatively short and the coatingrates are not optimal for industrial processes.

Nowadays, by contrast, the PVD processes, in particular sputteringprocesses, are distinguished by the fact that a wide range of coatablesubstrate materials are possible, that there is a virtually unlimitedchoice of coating materials, that the substrate temperature can beselected as desired, that the layer bonding is excellent and that it iseasy to influence the microstructure of the layers by selecting theprocess parameters. The drawbacks of the sputtering processes which wereoriginally developed have been substantially eliminated by extensivedevelopment work, and consequently nowadays sputtering technology isamong the most universal and widespread coating processes.

In recent years, the use and further development of magnetron sputteringsources has meant that in particular the magnetron sputtering processeshave proven suitable for industrial coating processes. The magnetronsputtering processes allow high coating rates in a low pressure range(down to under 0.1 Pa) with little heating of the substrate.

The procedure used in sputtering is fundamentally known to the personskilled in the art.

Substrates are coated by cathode atomization, preferably by magnetroncathode atomization, by means of a sputtering apparatus as described,for example, in DE 41 06 770. Targets, as they are known, of the layerstarting material are exposed to the action of a plasma cloud whichforms between two electrodes, with the target simultaneously forming thecathode. The atomized target material has an affinity for the reactivegas and, by forming a chemical compound with the latter, is deposited asa layer on the substrate.

EP 0 516 436 B1 has described a magnetron sputtering installation forcoating a substrate with one or more layers.

The specific form of the installation contributes to a more efficientsputtering process. For this purpose, the installation has a vacuumchamber in which a substrate holder in drum form and, at the walls ofthe vacuum chamber, targets of the layer starting materials are located,the targets being arranged on magnetrons.

Rotation of the drum on which the substrates are located causes thesubstrates to be uniformly coated. This form of sputtering also allowsthe substrates to be coated with a plurality of layers without having tobe taken out of the vacuum chamber, and the thickness of the layer issimple to influence.

However, it has been found that the known sputtering processes forcoating substrates with in particular optical layers and layer systemsstill lead to quality problems in the form of turbidity and relativelyextensive roughness of the layer surfaces, which can generally berecognized as regions with diffuse light scattering when the coatedsubstrate is illuminated. In addition to having a purely cosmeticeffect, turbidity also reduces the reflectance of the coating andtherefore the quality of reflection optics. In the case of filteroptics, this turbidity leads to a reduction in the transmittance. Inboth cases, in addition to the effect of light scattering, increasedabsorption can also contribute to reducing the product quality.

Therefore, the invention is based on the object of providing a processfor coating substrates which can be used to apply layers and layersystems which have a high optical quality and/or a high surfacesmoothness, and of providing coated substrates with a high opticalquality and/or a high surface smoothness.

This object is very surprisingly achieved by a process as described inclaims 1 to 10 and a coated substrate as described in claims 11 to 23.

Accordingly, the process according to the invention for coating asubstrate with at least one functional layer comprises the steps ofproviding a substrate in a vacuum system and providing the layerstarting material in this vacuum system, and coating the substrate bymeans of sputtering of the layer starting material, wherein thesputtering process for coating the substrate with the functional layeris interrupted at least once by the application of an intermediate layerwhich is very thin compared to the functional layer, is different thanthe functional layer and remains below a thickness of 20 nm.

The term sputtering is used to describe processes in which the layerstarting materials, which are in solid form as a target, are exposed tothe bombardment of ions, so that atoms, atom clusters or molecules ofthe target are emitted and as a result the layer starting material isatomized.

The functional layers, as they are referred to below in the presenttext, may be individual layers of a coating which are optically active(i.e. they have a function in terms of their effect on the radiationwithin a defined region of the electromagnetic spectrum). In this case,the coating may comprise just one functional layer or a plurality offunctional layers, for example an interference layer system made up offunctional layers with a high refractive index and a low refractiveindex.

The process described here for coating a substrate with at least onefunctional layer advantageously does not influence either the design ofknown installations or the known sputtering processes per se, but ratherrepresents a new process sequence used for the production of functionallayers, with the result that the quality thereof can be significantlyimproved. This does not require any changes to be made to installationswhich are known per se, but rather merely reorganizes the processsequence in accordance with the process according to the invention usingmeans which are known per se. The process is not restricted to specificsputtering installations, but rather can be transferred to any form ofinstallation which allows the sputtering of layer starting materials.

Magnetron sputtering has proven particularly advantageous, since it ispossible to achieve higher sputtering rates than with other sputteringprocesses and therefore there is an economic benefit.

In an advantageous embodiment, the substrate is coated in such a mannerthat functional layers with a low refractive index and functional layerswith a high refractive index are applied alternately by means ofsputtering in a reactive atmosphere. In this case, the functional layerswith a low refractive index preferably consist of SiO₂ and thefunctional layers with a high refractive index preferably consist ofZrO₂, since these materials are particularly suitable for opticalinterference systems.

The inventors have discovered that it is possible to achieve functionallayers of significantly reduced turbidity if the functional layers witha high refractive index formed from ZrO₂ are interrupted by very thinintermediate layers with a low refractive index formed from SiO₂.

Depending on the thickness of the functional layer, these intermediatelayers have a thickness of 0.1 nm-20 nm, preferably 0.5 nm-10 nm,particularly preferably 1 nm-3 nm, and are optically inactive, i.e. inany event they remain below a thickness at which they cause anysignificant changes to the spectral curve within the region of theelectromagnetic spectrum which is under consideration.

The functional layers produced using this process appear more brilliantand smoother and increase the transmittance and/or reflectance.

The process according to the invention is also suitable for coating asubstrate with a metal layer, in particular a functional layer ofchromium. In this case, the functional layer of metal, in particular ofchromium, is interrupted by the introduction of an oxygen-rich microwaveplasma, which can be generated by means of a microwave applicator. Inthis case, the process of sputtering pure metal targets or the Crtargets is briefly interrupted and the microwave applicator activated,which leads to oxygen being introduced into the vacuum. This oxygenreacts with the metal layer which has already been applied to thesubstrate, to form a thin metal oxide layer, and therefore forms a verythin intermediate layer. The sputtering of the metal or chromium targetis then continued. Layers produced in this manner have a significantlysmoother surface, which likewise contributes to better opticalproperties, and is also of benefit for further processing.

The inventors have discovered that this process leads to the measuredroughness of a surface of a chromium layer produced using this processbeing only half the roughness, measured in the case of a polishedstainless steel template, a previously preferred process for producinghighly polished electrically conductive surfaces.

Each of the abovementioned coating operations can be repeated anydesired number of times in order to obtain a plurality of functionallayers with intermediate layers; it is not necessarily imperative foreach functional layer to be interrupted by means of an intermediatelayer.

It is advantageous for the substrates to be fitted to a drum which islocated within the vacuum chamber and to rotate past the targetscomprising the layer starting materials and past the oxygen source. Thisensures homogenous coating.

It is obvious to the person skilled in the art that it is also possiblefor other suitable apparatuses to be used for the coating operation.

In addition to the process according to the invention, the inventionalso encompasses a coated substrate having at least one functional layerformed from a metal, in which the functional layer is interrupted atleast once by an intermediate layer, the intermediate layer consistingof a metal oxide and remaining at a thickness of less than 10 nm.

In particular for substrates whose functional layer is a chromium layer,it has proven beneficial to the smoothness of the surface for thisfunctional layer to be interrupted by means of an intermediate layer ofa metal oxide, in particular by means of a chromium oxide layer.

Substrates which have been coated with chromium in this manner are used,for example, as substrates for lithographic processes.

A further coated substrate according to the invention is intended foruse as an optical element, such as for example a color filter, fordigital projection.

The coating of the substrate in this case comprises at least onefunctional layer of a metal oxide, and at least one intermediate layerof a metal oxide which interrupts the functional layer. In this case,the thickness of the intermediate layer remains below a thickness atwhich it is optically active.

The individual functional layers are preferably functional layers with alow refractive index and functional layers with a high refractive index,the functional layers being interrupted by at least one intermediatelayer of a metal oxide. In this case, an intermediate layer with a lowrefractive index formed from SiO₂ is introduced into a functional layerwith a high refractive index formed from ZrO₂.

Since coatings of substrates configured in this manner have theabovementioned good optical properties, they are used in numeroussectors.

Substrates which have been coated in this manner are not tied to knownsputtering processes, and it is also conceivable for them to be producedusing other processes, for example using CVD processes.

The processes disclosed here merely represent possible advantageousprocesses by means of which coated substrates according to the inventioncan be produced.

The invention is explained in more detail below on the basis ofpreferred embodiments and with reference to the appended figures, inwhich identical reference symbols denote identical or similar parts. Inthe drawings:

FIG. 1 diagrammatically depicts a magnetron sputtering device forcoating substrates with SiO₂ and/or ZrO₂.

FIG. 2 diagrammatically depicts a magnetron sputtering device forcoating substrates with Cr.

FIG. 3 diagrammatically depicts a cross section through a layer systemin accordance with one embodiment of the invention.

The illustrations are not to scale; the thickness of the layers and theratio of the layer thicknesses with respect to one another can inprinciple be selected as desired for the particular application.

EXEMPLARY EMBODIMENTS

FIG. 1 shows a diagrammatically illustrated magnetron sputtering devicewhich can be used for the coating of substrates with functional layerswith high and low refractive indices.

The basic structure of a magnetron sputtering device of this type isknown from EP 0 516 436 B1, and consequently will not be described inmore detail in the text which follows.

Inside the vacuum chamber (5) there is a drum (7), to which theindividual substrates (1) that are to be coated are secured.Furthermore, the magnetron sputtering device illustrated in thisexemplary embodiment has four sputtering electrode units (10 a, 11 a),as well as a pump (9) and two microwave generators (8), distributed overits circular wall (6). It has been found that the installation describedin EP 0 516 436 B1 is eminently suitable for coating a substrate inaccordance with the invention, but the process is not restricted to thisspecific installation, but rather can also be carried out on othersputtering installations.

In a preferred embodiment for the production of blue filters with metaloxide layers, a plurality of substrates (1) are placed onto the drum (7)inside the vacuum chamber.

To coat these substrates (1) with an alternating layer system made up ofZrO₂ with a high refractive index and SiO₂ with a low refractive index,with the first ZrO₂ layer having a thickness of approx. 93.3 nm, Zratoms are introduced into the vacuum chamber (5) by sputtering of the Zrtargets (10 b), and these Zr atoms react with the reactive oxygen gasthat has been admitted from the microwave generators (8) to form ZrO₂,forming a first sublayer of the functional layer at a coating rate of14.1 nm of ZrO₂/min after 205 s. This is followed, for a short period of4 s, by reactive sputtering of Si atoms from the Si targets (11 b). TheSiO₂ which is formed is deposited as an intermediate layer, at a coatingrate of 25.7 nm of SiO₂/min, on the first ZrO₂ functional layer applied.

The short coating duration of just 4 s in this case for the applicationof the intermediate layer of SiO₂ results in a calculated thickness ofthe intermediate layer of 1.7 nm.

Then, Zr from the Zr targets (10 b) is then sputtered again in areactive atmosphere for 192 s to produce the as yet absent second halfof the first functional layer.

In the next step, a further functional layer with a low refractive indexis applied. This layer consists of silicon oxide, which is reactivelysputtered into the vacuum chamber (5) from the Si targets (11 b) as Siatoms with a coating rate of 25.7 nm of SiO₂/min. This SiO₂ is likewisedeposited on the layers that have previously been applied. In this steptoo, the duration of the coating operation depends on the thickness ofthe layer which is to be applied.

It is obvious that the functional layer formed from SiO₂, if desired,can also be split by a very thin intermediate layer formed from ZrO₂.

Depending on the particular application, it may be necessary to apply aplurality of alternating layer systems to achieve the intended opticaleffect of an alternating layer system, e.g. of a blue filter. It is alsoquite obvious for a plurality of layers to be divided in this mannerwithin an alternating layer system of this type.

FIG. 2 shows a further embodiment of a magnetron sputtering installationwhich is used to produce chromium layers in accordance with theinvention on substrates for lithographic processes. In terms of itsstructure, it corresponds to the magnetron sputtering installation shownin FIG. 1, but in this case has only two sputtering electrode units (12a).

The substrates (1) are provided on the drum (7) inside the vacuumchamber (5). To apply a first functional layer of chromium to asubstrate (1) Cr atoms are introduced into the vacuum chamber (5) bymetallic sputtering of the Cr targets (12 b).

In this respect, it is crucial that there is no oxygen in the vacuumchamber (5) and that no oxygen is supplied. The sputtering process iscarried out until the desired thickness of the chromium layer, in thiscase 30 nm, has been reached. Then, the sputtering electrode units (12a) are switched off and the microwave generators (8) are brieflyactivated, resulting in the formation of an oxygen plasma in the vacuumchamber (5), which partially oxidizes the freshly sputtered metallicchromium surface. The thickness of the oxide layer formed is so thinthat it has no influence on the spectral properties, in particular thereflection properties, of the mirror layer.

After this operation, the microwave generators (8) are switched off andthe sputtering electrode units (12 a) are activated again, so that afurther layer of chromium of approx. 30 nm is applied by metallicsputtering of the Cr targets (12 b). This procedure is repeated untilthe total desired thickness of 270 nm has been reached.

FIG. 3 diagrammatically depicts a substrate which has been coated with afunctional layer (2) in accordance with the process described above. Inthis case, a first half of a functional layer (3) has been applied tothe substrate (1), followed by interruption with an intermediate layer(4), and then the second half of a functional layer (3) has been appliedto the intermediate layer (4). Depending on the particular applicationand on the stipulated optical design, it is quite obvious for aplurality of functional layers (2), including different functionallayers, which have been divided in this manner to be applied on top ofone another.

1. A process for coating a substrate with a functional layer, comprisingthe steps of: providing the substrate and a layer starting material in avacuum system; sputtering the layer starting material on the substrateto define a first portion of the functional layer; interrupting thesputtering at least once to produce an intermediate layer on the firstportion, the intermediate layer being different than the functionallayer and having a thickness of less than or equal to 20 nm; andcontinuing sputtering the layer starting material after the intermediatelayer is produced to define a second portion of the functional layer,wherein the intermediate layer is sufficient to increase thetransmittance and/or reflectance of the functional layer.
 2. The processfor coating a substrate as claimed in claim 1, wherein the sputteringcomprises magnetron sputtering of the layer starting material. 3.(canceled)
 4. The process for coating a substrate as claimed in claim 1,further comprising repeating the sputtering, interrupting, andcontinuing steps so that a plurality of functional layers are applied asan alternating layer system comprising a first functional layer with alow refractive index and a second functional layer with a highrefractive index.
 5. The process for coating a substrate as claimed inclaim 4, wherein the first functional layer has a first intermediatelayers with a high refractive index and/or the second functional layerhas a second intermediate layer with a low refractive index.
 6. Theprocess for coating a substrate as claimed in claim 5, wherein the firstfunctional layer and the second intermediate layer consist of SiO₂ byvirtue of silicon being sputtered in a reactive atmosphere, and thesecond functional layer and the first intermediate layer consist of ZrO₂by virtue of zirconium being sputtered in a reactive atmosphere.
 7. Theprocess for coating a substrate as claimed in claim 1, wherein the layerstarting material comprises a pure metal target.
 8. The process forcoating a substrate as claimed in claim 7, wherein the interrupting stepcomprises introducing an oxygen-rich microwave plasma into the vacuumchamber so that a surface of the first portion of the functional layeris oxidized.
 9. The process for coating a substrate as claimed in claim8, wherein the pure metal target comprises chromium.
 10. The process forcoating a substrate as claimed in claim 1, further comprising locating aplurality of substrates, on a drum inside the vacuum chamber androtating the drum so that the plurality of substrates rotate past aplurality of targets comprising the layer starting material and anoxygen source.
 11. A coated substrate comprising: at least onefunctional layer of a metal; and at least one intermediate layer of ametal oxide which interrupts the at least one functional layer and has athickness that is less than or equal to 10 nm.
 12. The coated substrateas claimed in claim 11, wherein the at least one functional layer is achromium layer.
 13. The coated substrate as claimed in claim 12, whereinthe at least one intermediate layer is at least one chromium oxidelayer.
 14. (canceled)
 15. The coated substrate as claimed in claim 11,wherein the coated substrate is used as a substrate for lithographicprocesses.
 16. A coated substrate comprising: at least one functionallayer of a metal oxide; and at least one intermediate layer of a metaloxide which interrupts the at least one functional layer and remainsbelow a thickness at which the at least one intermediate layer isoptically active.
 17. The coated substrate as claimed in claim 16,wherein the at least one functional layer comprises an alternating layersystem made up of a first functional layers with a high refractive indexand a second functional layers with a low refractive index.
 18. Thecoated substrate as claimed in claim 17, wherein the second functionallayer is formed from SiO₂ and the first functional layer is formed fromZrO₂.
 19. The coated substrate as claimed in claim 18, wherein the atleast one intermediate layer in the first functional layer has a lowrefractive index formed from SiO₂, and the at least one interruptingintermediate layer in the second functional layer layer (4) with has ahigh refractive index formed from ZrO₂.
 20. (canceled)
 21. The coatedsubstrate as claimed in claim 16, wherein the coated substrate is usedas an optical element.
 22. The coated substrate as claimed in claim 21,which is used as wherein the optical element is a color filter.
 23. Thecoated substrate as claimed in claim 16, wherein the at least onefunctional layer is an optical functional layer.
 24. The process forcoating a substrate as claimed in claim 5, wherein the first and secondintermediate layers have 0 a thickness of less than or equal to 10 nm.